1. Field of the Invention
The present invention relates in general to aircraft and more particularly to an aircraft that includes a variable forward-sweep wing.
2. Background Art
Modern military aircraft must be versatile. Typically, the same aircraft may be required to act as a bomber aircraft and deliver a large weapons payload to an enemy target while in the next mission be required to assume a fighter aircraft role and engage enemy planes. These varying roles require the aircraft to be able to operate in several flight regimes. For example, the aircraft must be able to operate at speeds below the speed of sound (subsonic), at or near the speed of sound (transonic) and above the speed of sound (supersonic). Moreover, the aircraft must be highly maneuverable at all these speeds, have good handling qualities and be fuel efficient. One critical factor in the ability of the aircraft to meet these varied goals is the design of its wing.
Generally, an aircraft wing is either unswept or swept. An unswept wing, or a wing whose leading edge is approximately perpendicular to the centerline of the fuselage, is typically best suited for aircraft flying at subsonic speeds. Furthermore, the unswept wing allows the aircraft to land on shorter runways such as presented by aircraft carriers and to carry heavier loads.
One problem, however, with the unswept wing is that its transonic and supersonic characteristics are unsatisfactory. This is because, as the speed of the aircraft increases to transonic and supersonic, there is a dramatic increase in drag on the unswept wing preventing the aircraft from accelerating further. In addition, the handling, the control and the stability characteristics of the aircraft are drastically altered. The unswept wing is unable to overcome these undesirable characteristics at transonic and supersonic speeds.
On the other hand, a swept wing is typically best for aircraft flying at transonic or supersonic speeds. A swept wing is a wing whose leading edge is at an angle to the fuselage centerline. The swept wing, having a low wave drag and a low aspect ratio, allows an aircraft to fly efficiently at transonic and supersonic speeds. Moreover, a swept wing allows the aircraft to penetrate smoothly through the transonic speed regime. Despite these advantages, however, the swept wing is generally less desirable for use at subsonic speeds.
A swept wing can be swept either backward or forward. A backward-sweep wing is characterized by having its wing root located predominately forward of its wing tip. This means that the root of the wing is closer to the front of the aircraft than the tip of the wing. In other words, the backward-sweep wing is swept back away from the direction of flight.
Conversely, a forward-sweep wing is characterized by wing tips that are located predominately ahead of the wing root. This means that the forward-sweep wing is swept forward in the direction of flight. A forward-sweep wing is much less common than the backward-sweep wing because of its unfavorable aeroelastic properties.
One problem with the backward-sweep wing is that a stall tends to occur first at the wing tips and then move inboard toward the wing root. A stall occurs when the air over the wing separates from the wing surface and eventually leads to loss of lift. This wing tip flow separation on a backward-sweep wing causes the aircraft to pitch up. In addition, roll control is lost in a stall since the roll control surfaces are located near the wing tips.
One problem the forward-sweep wing has in common with the backward sweep wing is that they both do not perform well at subsonic speeds. As mentioned previously, a swept wing, whether backward-sweep or forward-sweep, is generally unsatisfactory for flight at subsonic speeds and its operation on short runways or to carry heavy loads is restricted.
Therefore, what is needed is an aircraft capable of subsonic, transonic and supersonic speeds that maintains desirable handling, control and stability characteristics at all these speeds. The aircraft should be capable of landing on short runways, carrying heavy loads, overcoming the increased drag at transonic speeds and flying at supersonic speeds.
What is further needed is an aircraft that will retain roll control in a stall by avoiding wing tip stalls. Additionally, what is further needed is an aircraft that can cruise at transonic and supersonic speeds without unnecessary drag that can increase engine power and fuel requirements. Moreover, a further need is an aircraft that is highly maneuverable at all speeds.
Whatever the merits of existing and the above-mentioned aircraft, they do not achieve the benefits of the present invention.